Haemophilus parasuis is an important cause of mortality in nursery pigs (Oliveira and Pijoan, 2004, Vet. Microbiol., 99(1):1-12). It is a commensal organism that colonizes virtually 100% of swine herds worldwide, and a frequent opportunistic pathogen during concurrent viral infections and stressful events (Oliveira et al., 2004, Safety of Controlled exposure to Haemophilus parasuis: the role of sow vaccination and PRRS virus infection. Proceedings of the 18th International Pig Veterinary Congress, Hamburg, Germany, p. 189), causing up to 60% mortality in naïve populations.
Protection against H. parasuis systemic infection is associated with the presence of antibodies against this pathogen (Solano-Aguilar et al., 1999, Am. J. Vet. Res., 60(1):81-87). The high susceptibility of colostrum-deprived pigs to H. parasuis systemic infection corroborates the importance of maternal antibodies as a major defense mechanism against this agent (Oliveira et al., 2003, Can. J. Vet. Res., 67(2):146-150). Commercial and autogenous vaccines currently available for H. parasuis generally do not provide complete cross-protection among different strains and serotypes due to extensive genotypic and phenotypic variation between different H. parasuis strains (Oliveira et al., 2003, Am. J. Vet. Res., 64(4):435-442, Oliveira et al., 2004, J. Swine Health and Production, 12(3):123-128). This is a major limitation to preventing nursery mortality against this pathogen. Antibiotics can be useful in controlling H. parasuis outbreaks; however, like other modern food-animal based industries, the swine industry is rapidly reducing the use of antibiotics to control mortality.